1. We investigated the effects of a wildfire on stream physical, chemical and biological characteristics in a Mediterranean climate, comparing stream community structure and consumer resource use in burned versus unburned catchments in Santa Barbara County, CA, U.S.A. 2. Canopy cover was lower and water temperature was higher in streams draining basins where the riparian vegetation burned than in streams in unburned basins or burned basins where riparian vegetation remained intact. Stream flow and suspended sediment concentrations during large post-fire storms and wet season nutrient levels were higher in burned than unburned catchments, with increased sedimentation after flood peaks. 3. A year after fires, algal levels were highest in streams where riparian vegetation burned and lowest in streams in burned basins where the riparian canopy remained intact. In contrast, streams in burned basins had lower particulate organic matter, detritivore and predator levels than unburned basins, regardless of whether riparian vegetation burned. Where present, southern California steelhead trout (Oncorhynchus mykiss) were extirpated from burned basins. 4. Algivore densities were high in streams with burned riparian vegetation for two post-fire years before declining to unburned stream levels. Shredder densities rebounded in streams in burned basins with intact riparian vegetation, but remained low for 4 years where riparian vegetation burned. Predatory invertebrate densities increased at sites where trout were eliminated by wildfire. 5. Hydrogen stable isotope analysis indicated that the diets of most invertebrate taxa in streams with burned riparian vegetation a year after fires were comprised of a higher proportion of algal material than riparian detritus relative to invertebrates in streams with intact riparian vegetation. 6. Wildfire impacts on stream food webs are determined, in part, by fire severity in the riparian zone. Streams with burned riparian canopies supported algal-based food webs and streams with intact riparian canopies sustained detrital-based food webs. Fire affected basal resources (nutrients, light, allochthonous inputs) with bottom-up effects on primary producers and consumers, but top-down effects were decoupled at the trophic link between invertebrate predators and primary consumers.

In stream ecosystems, the growth of aquatic primary producers is affected by spatial and temporal variations in the riparian canopy, which can influence the availability of light resources. Aquatic plants can acclimate to low light environments by employing a suite of morphological or physiological mechanisms to increase light capture or photosynthetic efficiency. Some species may also use alternate types of propagules to colonize environments with heterogeneous light environments. In a greenhouse experiment we examined the morphological and physiological response of watercress (Nasturtium officinale R. Br.) to a gradient of increasing light levels, which ranged from 7% ambient light to full sunlight. We also determined if watercress seedlings and vegetative fragments differed in their growth response to increasing light levels. Total biomass and root biomass of seedlings and vegetative fragments decreased with decreasing light levels. The difference in plant biomass across treatments was due to morphological changes in total canopy area and leaf area, both of which increased with decreasing light levels. Seedlings and vegetative fragments did not differ in their response to light availability, but vegetative fragments had higher final biomass as a result of higher initial biomass. Physiological acclimation to low light levels appears to be of secondary importance for watercress as the concentrations of total chlorophyll, chlorophyll a, chlorophyll b, and chlorophyll a:b did not differ among light levels or between seedlings and vegetative fragments. Seedlings and vegetative fragments grown under high light levels had a greater percentage of carbon and a lower percentage of nitrogen than plants grown under low light conditions. The results of this study indicate that watercress displays considerable morphological plasticity and acclimates to low light conditions primarily by increasing leaf area and canopy surface area. There is no evidence that the type of watercress propagule (seedling vs. vegetative fragment) imparts any growth advantage in low light environments and watercress grown from either type of propagule showed no differences in their morphological or physiological responses to varying light regimes.

We compared the efficacy of stable carbon, hydrogen, and nitrogen isotope ratios in identifying the resources used by insect consumers in headwater streams of southern California. We also compared gut contents with consumer stable isotope ratios and mixing model estimates of resource contributions to predator diet. Stable hydrogen isotope ratios (as δ2H) of algivores were well separated from ratios for detritivores, whereas relationships between stable carbon (as δ13C) and nitrogen (as δ15N) ratios of consumers and their expected diets were weaker and more ambiguous. δ2H values of primary consumers more strongly reflected the proportions of their gut contents consisting of algae than δ13C values. Τhe proportions of algivorous prey in predator gut contents increased with mixing model estimates of algivore contributions to predator diet using δ2H but not δ13C values. Our findings support the use of hydrogen isotope ratios in food web studies of streams in southern California and their potential use in assessing the effects of anthropogenic and natural disturbance on basal resource contributions to food webs that might not otherwise be identified using carbon isotope ratios.

No abstract

The mechanisms that allow broadly distributed aquatic plants to inhabit variable resource environments are unclear, yet understanding these mechanisms is important because broad environmental tolerance is often linked to invasiveness in terrestrial and aquatic plants. In an experimental stream, we examined the effects of different nutrient concentrations on the growth rate, biomass, and foliar nutrient concentrations of a cosmopolitan and potentially invasive aquatic plant, Nasturtium officinale (R. Br.). Nasturtium seedlings were grown under six nutrient treatment levels ranging from 0.64 lm N:0.09 lm P to 1531 lm N:204.13 lm P, for 8 weeks. Absolute and relative growth rates, and biomass of seedlings increased along a gradient of increasing nutrient concentrations but the effect of nutrient concentration was dependent on growing time.Seedling biomass varied among nutrient treatments in weeks 4 through 8 of the experiment, but did not differ in week 2. By week 8, the two highest nutrient treatments had greater biomass than the two lowest nutrient treatments. Foliar nitrogen concentration increased, whereas carbon concentration and C:N ratios decreased in response to increasing nutrients. Nasturtium grows slowly in nutrient-poor conditions but rapidly increases its growth, biomass accrual, and nitrogen storage as conditions become nutrient-rich. The response of Nasturtium to enhanced nutrient conditions may indicate how aquatic nuisance species successfully invade and dominate plant communities in streams, where resources often vary both temporally and spatially.